We are using diploid human lymphoblasts as target cells in an assay for chemical mutation of the HGPRT genetic locus. The system has been developed to the point that dose and time dependence studies are routine. Very recently we have been able to prepare sterile rat liver post mitochondrial supernatant fractions compatible with the lymphoblasts and active in drug metabolism. Our studies of chemically induced neoplastic transformation in a C3H mouse cell line are continuing in the direction of characterizing the time parameters of fixation and the beginning of neoplastic cell division. We are also studying some interesting effects of caffeine on the transformation process.